>When considering 2-stroke versus 4-stroke engines, equivalences seem to be
>based on a torque comparison. For instance, an .80 ci 4-stroke may be
>recommended as an equivalent to a .60 ci 2-stroke.
>Since power (nominally the rate of application of torque) is what enables
>the plane to fly, why aren't the engine comparisons based on horsepower?
>I'm thinking of putting a 4-stroke in a Goldberg Tiger 60, and they
>recommend a .45 to .60 2 cycle or a .65 to .80 4-cycle and this seems to be
>consistant with engine performance charts I've seen. It looks to me like a
>.7 4-stroke may be rated at 1.1 HP whereas a .46 2-stoke may be rated at
>about 1.6 HP. A .9 4-stroke may put out about 1.6 HP as opposed to 1.6 HP
>for a .6 2-stroke.
>What is it that I'm missing here?
Good analysis, but IMHO the first assumption is probably incorrect. I
think most modelers consider equivalence of the engine types to mean
what produces equivalent performance, and as you observed, power is
what enables the plane to fly. The forte of 4-stroke engines is the
ability to produce good power in lower rpm ranges that are usable in
model aircraft. To produce the same power at lower rpm, obviously,
according to the relationship between power and torque you cited, they
must produce more torque than their 2-stroke 'equals.'
Your Tiger 60 is a good platform for an example.
With a Saito .65 or .72 -stroke and APC 11X8, rpm will be 10,500 -
11,000. Thrust is in the range of 4.7 - 5 lb, and speed potential
about 80 mph. This is at a power level of .95 - 1.1 bhp, which these
engines can produce comfortably.
With a .46 2-stroke that is hot (capable of producing actual power
comparable to the 1.1 bhp produced by the Saito .72 - the 1.6 hp
figure is ad man BS), a 9X7 prop will turn at 15,000+, thrust is in
the same ballpark at about 4.5 lb, and speed potential is near 100
Overall the performance is about same. Though speed potential is
higher for the .46 2-stroke in this example, it will never be reached
using either engine on the Tiger 60, which will likely top out at
about 60 mph. The most significant difference to me is that at WOT
the Saito is running at a comfortable rpm and the noise level produced
sites. The .46 is screaming at near racing rpms, and prop generated
noise level is twice at great, 96+ dBA, which is not acceptable at
BTW, a .46 that is actually capable of 1.6 bhp will turn that 9X7 APC
smallest recommended prop is a 10.5X6, so I'll use that. At 1.6 bhp,
the .46 FX should turn that prop at 14,500. Wanna bet it will?